Cryptographic communications system



June11,1946. J. E. Smm, l 1 2,401,888

CRYPTOGRAPH-IC COMMUNICATIONS SYSTEM Filed June 19, 1942 ATTORNEY Patented june il, 1946 caw'rooaarnrc connuUNrcA'rroNs SYSTEM Application .lune 19, 1942, Serial No. 447,657

(Cl. 17g-1.5)

l2 Claims. l

This invention relates to cryptographic com-a munications systems,- and more particularly to a device for enciphering and deci-phering a train of signals by chronologically transposing certain elemental portions of such a train.

In the present invention, I have devised an enciphering mechanism which stores a record of speech or some other train of signals, such, for example, as telegraph code signals or facsimile signals. The stored signal train consists of elemental portions which are transposed according to a predetermined pattern, so that the message is substantially completely scrambled and rendered unintelligible when transmitted over a communications channel. At an authorized receiving station intelligibility is restored by again transposing the same elemental portions of the signal train. This operation is performed by a deciphering mechanism which is similar in every respect to that of the enci-phering mechanism at the transmitting station. At the two stations, one for transmission and the other for reception, means are provided for frequently varying the pattern in accordance with which the elemental portions of the signal train are transposed. At a given moment, however, the same transposition pattern must be effective at the two stations, otherwise intelligibility will not be restored at the point of reception.

In carrying out my invention, .continuous broken down into discrete blocks which are rearranged in a different chronological order,

It is another object of my invention to provide a system for re-arranging the blocks of intellitransmission of the intelligence, whether it be speech or a train offacsimile signals, is rendered possible by breaking downthe normal train of signals into discreet signal-time intervals before transmission and re-arranging these signal-time intervals or blocks of intelligence in a different chronological order. Signal-time intervals are purposely chosen to be on the order of from 50 to 100 milliseconds in duration. Such vintervals have been found to produce complete confusion a single syllable into a number of parts which, when transposed, completely obliterate the original sound. When the invention is used for the transmission of picture signals, however, the duration of each block of intelligence depends upon the transmission time for an elemental area of the picture. Several elemental picture areas may be included in one bloclr, but, for the pur pose of eectively scrambling the picture signals. the transposition of blocks in adjacent scanning lines should follow no uniform pattern.

It is an object of my invention to provide a cryptographic transmission system in which the scrambling of a signal train shall be performed -40 of speech`since they result in the breaking up of gence as re-recorded on a magnetic tape in such manner that deciphering of this intelligence when transmitted and intercepted at an unauthorized point of reception shall be rendered extremely difficult. I

It is still anotherobject of my invention to provide a magnetic tape recording system for the storing, subsequent transmission, reception, and declphering of cryptographic messages, this system being arranged for varying the transposition pattern from time to time with respect to certain blocks of signals, said pattern being in accordance with a predetermined code which is available only to the authorized sending and receiving stations.

I am aware that various privacy methods of transmission of intelligence are known in the art, but it appears that they have many disadvantages and defects. In carrying out my invention, I have endeavored to devise a system which shall be free from such `disadvantages and defects. I, therefore, have arranged my system so that it shall not require a wide frequency band for transmission over a radio channel. My system is also arranged so that the radio channel does not need to be used for transmission of a decoding key. Furthermore, the transmission of intelligence by my system is effected at no loss of speed as compared with the transmission of intelligible signals. Then again, there is no wastage of power at the transmitter for superposing or interjecting spurious signals into the useful subject matter which is to be transmitted.

The frequency band required for transmission of cryptographic messages in accordance with my invention is identica1 with that which is required for normal transmission of uncoded-signals. This is an important feature, pantcularly when the system is used on circuits of limited band width or on long distance radio channels where selec tive fading i's found to be a limitingfactor. My invention will now be described in more detail, reference being made to the accompanying drawing, the sole gure of which represents diagrammatically the .essential components which are assembled for use at a transmitting station I or at a receiving station.

Referring to the drawing, Ishow for illustrative purposes, a microphone 8 which maybe con- 'nected through a plug 4 I and jack I0 to an am- 2plifier II.

The output from the amplifier is fed to a magnetic tape recording unit I2 which may be ofA conventional` design operating upon a strip of magnetic tape 5. This magnetic tape is preferably formed in a loop and is continuously moved in one direction, being stretched overtwo pulleys 5 and 1. Y

Pulley 6 is mounted on a shaft I9, which should be driven by a synchronous motor, not shown. The frequency of the power supply for'the motor should be maintained constant, as by means of any well-known system of tuning fork regulation.

'I'hc tape 5 is Preferably perforated with sprocket feed holes which mesh with sprocket teeth 45 on the periphery of the pulley 5. The tape velocity is thus maintained constant. At the transmitting and receiving stations separate tuning fork frequency standards, if they are nominally the same, may be used without resort to synchronizing methods. A certain phase adjustment must be made, however, between an enciphering cam at the transmitter and a deciphering cam at the receiver, as will be hereinafter described' in more detail.

In place vof the microphone 8, I may, if desired, substitute a facsimile picture scanner 9 for transmission of picture signals. The plug 42 at thev end of the output circuit from the facsimile scanner may, therefore, be inserted inthe jack I0. Under these conditions, the light and dark ele- 'mental areas ofv a picture may be scanned and.

signals formed which will be recorded continuously on the tape 5.

It is intended that substantially identical apparatus shall be used at sending and receiving stations. 'I'he drawing, therefore, shows essential equipment for use at either station, except as indicated by legends. Hence, a radio receiver 44 may be plugged into the jack I0 through the plug 43, when deciphering a cryptographic train A of signals at a receiving station.

Along one stretch of the tape 5, and spaced at uniform distances one from the other, is a number of individual pick-up units I, 2, 3, and 4. .The uniform spacing between the pick-up points is indicated by the dimension d. The distance between these pick-up points is preferably made to correspond'with a 90 arc on the periphery of the pulley 6. In this manner, the pulley 6 will rotate through exactly one quarter of a revolution in order to drive the tape from one pick-up point to the next, i. e., c :r the distance d. Each of the pick-up coils I, 2, 3, and 4 is connected to the input circuit of an amplifier I3 individual thereto.v The output circuits for the respective amplifiers I3 are .connected through a rotary switch I8 to individual coils I4, I5, I5, and I1 in a group of re-recording units. A variable pattern of transposition of the circuit connections between coils I, 2, 3, 4 and coils I4, I5, I5, I'I, is preferably produced by means of a four-bank rotary switch I8 wherein each' bank contains contacts in the four banks are individually la beled w, z, y, and z. Rotary switches of this type. are in common use-in aromatic telephone switching and for other purposes.

respective amplifiers I3. This source 24 is also 4 ing a matter which is reserved for further discussion hereinafter.

The amplifiers I3 are provided with a common cathode return circuit to` ground. 'Ihis return circuit is' connected to ground periodically by means of a rotary distributor switch mounted on lthe shaft I9 which also carries the pulley 5. A conductive segment 20 in this rotary distributor subtends an arc of 90. The remaining peripheral arc of .this distributor is insulated and measures 270. A contact spring or brush 2| engages with the peripheral segments of the distributor. A grounded contact spring or brush 22 engages with a collector ring 23 mounted on the shaft I9.

A direct current source 24 supplies operating potential through the re-recording magnets I4, I5, I5, and I'I to the tubes (not shown) in the adapted to supply a certain polarizing bias to the magnetic cores in the re-recording units so as to compensate for an opposing bias derived from the wipe-out magnetizing unit 25iwhich is disposed to the right of the pick-up unit group along the upper stretch of the magnetic tape 5.

Wipe-out unit 25 is continuously energized by means of a source 2B. This unit is effective to obliterate all traces of the signalsas recorded by the unit I2. A similar wipe-out unit 21 is arranged to clear the lower stretch of magnetic tape of all signals recorded thereon by the rerecording units I4, I5, I5, and I1. The wipe-out unit 21 is also continuously energized by means of a source 23.

A pick-up unit 29 is arranged to deliver signals' are used, as above mentioned, for permutation-v ally varying the chronological order in ,which different blocks of the signal train are re-recorded on the tape 5 by the re-recording units I4, I5, I5, and I'I. The total number of available permutations is theoretically factorial 4, if four pick-up units are used.

It is found, nevertheless, that a number of-these permutations are of little use, since they do not result in sufficient scrambling to obtain an unintelligible transmission. The eleven best transposition patterns for enciphering the signal blocks (where the-natural order of the blocks is 1, 2, 3, 4), are those which do not contain any normal sequence of any twoblocks. The chosen permupreferably eleven contact segments. The wiper` tations are as follows:

Operations at the transmitting station will now be described.

The recording unit I2 applies input signals continuously tothe moving tape 5. During one revolution of the shaft I9 a point on the Atape lmoves from the pole pieces of the recording unit I2 to the pole pieces of pick-up unit I. Since there are four pick-up umts which are arranged to be actuated simultaneously, the time of actuation is one-quarter ofthe total time, and is determined by the passage of the distributor segment 2li under the brush 2I.

The amplifiers I3 include discharge tubes (not shown) whose cathodes may be connected -to ground through a common circuit which includes the conductor $6 leading to contact 2i, conductive segment 20, collector ring 23, and the grounded brush contact 22. When contact 2l rests on the insulated segment of 270". the amplifiers i3 are inactive, since their cathodes have no return path to the negative side of source 24.

Instead of resorting to circuit-breaking of the common cathode return lead as described in the preceding paragraph, it is also feasible to control the times of operation of the amplifiers i3 by a shift in the grid bias potential applied to the control grids of their discharge tubes. This would involve a slight modification of the circuit arrangement which will be clearly understood by those skilled in the art. Specically, the control grid of the tube in each amplifier i3 may have a resistive connection through a biasingsource (not shown) the positive terminal of which would be connectable to ground through a rotary switch'having a closing period equal to three-quarters of a revolution of the shaft it. During this period, therefore, the amplifier tubes would be biased to cut-ofi and would be inactive. even though their cathodes were permanently grounded. During one-quarter of the total time, also, contact 2i, when used as a switch in the biasing circuit, would rest upon a 90 segment of insulation, thus removing the cut-off bias and enabling the input circuits of the tubes in amplifiers I3 to be fully controlled by magnetic flux variations in the cores of the pick-up units i, 2, il, and 4l.

During the intermittent activation periods of the amplifiers I3, the tape travels over the distance d. Four successive sections or blocks" of the signal train are thus sensed simultaneously by the pick-up units i, 2, 3, and d. Also, during the periods when the amplifiers i3 are not activated, the tape 5 moves a distance equal to 3d in order to avoid detection of any part of the signal train more than once.

The signals thus picked up. sensed, or detected, by magnetic induction in .the units i, 2, il, and d are amplified in the amplifiers is and delivered to the re-recording units it, i5, it. and i? in a re-arranged order, which order is dependent upon the position occupied by the rotary contact wipers of the switch i8. In the position in which 'these wipers are shown, the signals picked up by unit i are fed through wiper w to unit i5; signals picked up by unit 2 are fed through wiper :r to unit lli; signals picked up by unit 3 are fed through wiper z to unit l. As the re-recorded blocks are moved to the pick-up point of coil 29, therefore, the train of signals is sent out as though the blocks were arranged in the order 2, l, 4, 3. This order will be changed from time to time by stepping the wipers of switch i8 to different successive positions. Thus in thev position of the wipers following that which has been shown, and assuming that these wipers are moved counter-clockwise. the order in which the blocks of signals will be laid down on the magnetic tape by the re-recording unit it, l5, it, and il will correspond with pick-up units arranged in this order-l, 4, 3, 2.

. A phase relation must be maintained between the contact segments 2t at the sending and receiving stations in order that the breaking up of the signal train into transposable sections may be the same at the two stations. Accordingly, it is appropriate before sending a secret communication to take a few moments of transmission time for test purposes. During this time, ordinary speech may be transmitted, and if it is not intelligibly received, the shaft i9 at the receiver may be shifted in phase, as by rotating the field of its motor manually. The proper phase relation will be indicated by the reception of clean rather than guttural speech on a loud speaker or other responsive device.' After adjustment, this phase relation will be practically maintained for long periods of operation (all day, for example) by the use of tuning fork frequency control of the power for driving the motors.

For illustrative purposes, I have shown a device which is suitable for actuating the switch I8 at irregular time intervals which are integral multiples of a time unit measured by one revolution ofthe shaft I9. Each switch actuation is accomplished during a, time when the brush 2i is not in contact with segment 2t. It is. of course, preferable that switching clicks shall be avoided during the pick-up and re-recording of the intelligence blocks. For stepping operations applied to the switch i8, I preferably employ a stepping magnet 3l whose armature 32 is conventionally shownl as mechanically associated with a suitable device Ysuch as a ratchet and pawl system operating upon'/ the common shaft of the wiper contacts w. 1,11."

and 2. This construction is well known in the art `and does not, therefore, need to be illustrated or.;

A source 33,@ grounded at one terminal and feeding from the other terminal through the coil of stepping inag-A net 3l, is arranged in circuit with a contact spring 3d, where the latter is arranged to be contactedy described in this application.

at irregular intervals by humps 33 on a cam Sii. The cam. 35 may be driven by a reducing gear arrangement consisting of a gear` tt and a worm Si. The worm 3? is mounted on the shaft i9 in common with the pulley t. The humps 38- on the cam 35 are arranged to engage with the conta/rt spring @d at times selectively predetermined, but only when the contact 2i rests on an insulation portion of the distributor on shaft li). A small arc is subtended by each hump 33, so that the stepping magnet 3l may be impulsively energized and its armature 32 maybe released before contact El engages with its conductive segment 2G. Hence,`each switch step of the contact wipers w, :11, y, and z in the rotary switch it will be taken while in no way interfering with the pick-up and re-recording of the signal blocks.

The cam 3E is shown to have its humps t@ peripherally spaced apart by different angles.

The object of this arrangement is to provide a secret code in accordance with which the switchesV it at the transmitting station and at the receiving station shall be stepped forward always in phase agreement, and also at irregular time ,in-

tervals. The irregularity of switching renders the deciphering -of the cryptographic message transmission just that much more diicult when the message is intercepted at an unauthorized station.

With due regard for the requirements of main taining phase agreement between the switches l at the sending and receiving stations. it will be noted that not only must the same arrangement of humps 38 appear on the duplicate cams at the sending and receiving stations, but also these cams must be set at the same starting point :it the outset of transmitting a message. In order to eiect this setting, or phasing, of the cams, a particular setting time .may be agreed upon, `say,

at one minute past the hour and at any ten` minute interval thereafter. Once having adjusted 7 the cam 35 with its index mark 40 in a predetermined position, as against any suitable stationary index point which is the same at both stations,A

ation of the system. If by any chance a sety of cams were to be captured by an enemy, it would be only necessary to communicate between stations forthe lpurpose of cancelling their use. Another set of cams would then be made available in lieu of the captured cams.

Once having started the operation of the cryptographic system at the two stations, and having adjusted duplicate cams 35 at the two stations, so that their index marks 40 are in phase with each other, a test run of short duration would be required for phasing thev wipers w, x, y, and e in switch I8 at the receiving station with corresponding wipers in the switch I8 at the transmitting station. For` test purposes, ordinary conversation having no secret signicance may be scrambled in the usual manner at the transmitting station and then delivered to the modulator of the radio transmitter. The deciphering of the test signals as received would not take place, however, in the absence of correct phase adjustment of the wipers w, x, y, and z at the receiving station. Suitable deciphering of the scrambled speech would be obtained, however, after sulciently stepping the wipers of the switch i8 under control of a manual key 39. This key 3Q is arranged with a grounding contact for energizing the switch-stepping magnet 3| any number of times additional to the steps which would be produced by the humps 38 on the cam 35. As soon as phase adjustment of the wipers w, x, y, and z is reached, the intelligence of the scrambled speech is automatically restored, and it will then be known by such restoration of the intelligence that the phase adjustment has been brought about. Thereafter the tuning-fork control of the A. C. frequency used to drive the synchronous motor at each staftion will be suicient to maintain the switches I8 in step for any practical period of operation. During such period secret communications may be transmitted with full assurance that no unauthorized interception thereof would be of any ald or comfort to an enemy.

It will be seen that by the operation of the rotary switch I8 at irregular intervals in accordance with the position of the humps 38 on the cam 35, the permutation of transposed blockswhich exists during one period will be followed by an entirely different permutation, so as to avoid a continuous run over a long period of a single arrangement of the blocks such as might be decoded in an unauthorized manner. The

. every ten minutes.

switch I8 has different cross-connections for each of its steps, the chronological arrangement of the blocks is, therefore, re-arranged each time the switch I8 is actuated. The duration of the setting of the wipers on switch I8 at any particular point is determined by the time intervals between For the sake of simplicity in the drawing, a single-stage reducing gear has been shown for rotating the cam 35 at a slow speed relative to the speed of the shaft I9. Two or more stages of worm-gear reduction may, of course, be used if it is desired to actuate the rotary switch I8 a relatively small number of times per hour. Thus, for an example, which in no way limits the scope of the invention, the sprocket wheel l may rotate at R. P. M.; a first-stage worm gear having 50 teeth would rotate at 3 R. P. M.; and a second-stage worm gear having 30 teeth lwould'rotate once in ten minutes. The cam 35 is illustratively shown to have nine unevenly spaced humps 38. Such an arrangement would cause the rotary switch wipers to take nine steps But since there are eleven segmental contacts ln a switch bank of the type shown, the wipers must sweep nine times over the arcs of the stationary contacts while at the same time the cam 35 makes eleven revolutions. time interval so occupied would be 1 hr. 50 min. This would be the repetition rate for the schedule of time allotments covering successive steps of the switch I8, where the rest period of the Wipers on each stationary contact would have nine degrees of variability. A further factor of variability is, of course, obtained by substituting one cam 35 for another on different days, as previous'ly mentioned.

While I have illustrated my invention as though it required four pick-up units I, 2, 3, and I, it will be apparent that a different number of pickplicable particularly to the enciphering of messages such as speech or facsimile signals, it will be apparent to those skilled in the art that the invention itself is capable of numerous other applications Without departing from the spirit thereof. The scope of the claims is, therefore, not necessarily limited to the precise details of construction herein shown and described.

I claim:

1. A cryptographic system comprising a linear signal storage medium continuously movable past a point o f signal recording and past successive points of signal sensing, intermittently operable means for simultaneously sensing different blocks of signals at each of said signal sensing points, means controlled by said sensing means for rerecording said blocks of signals in a transposed order, and an outgoing signal circuit connected to a signal sensing means which is suitably positioned to continuously sense said blocks of signals in their transposed order.

2. A cryptographic system according to claim l and including switching means operable at predetermined intervals for varying the order of transposition of said blocks of signals..

3. A cryptographic system according to claim 1 in which said storage medium is constituted by a magnetizable tape, said signal sensing means is constituted by a plurality of magnetic pick-up units, and said re-recording means is constituted by a plurality of magnetic recording u nits.

4. A cryptographic system according to claim l and including switching means operable to render said signal sensing means effective for such in- The ascisse termittent time intervals that a continuously recorded train of signals applied to said storage medium is completely sensed without overlapping.

5. in a signal storage system, a linear storage medium continuously movable past a Point 0f signal recording and past successive equally spaced points of signal pick-up, a signal recording unit effective at one of said points to spread a continuous train of signals on said medium, a plurality of pick-up units disposed at said equally spaced points along said medium, a plurality of re-recording units also disposed along said medium at other points equidlstant from each other, a, final pick-up unit positioned along said medium beyond the re-recording units, two erasing units to one of which the originally recorded signal train is fed after passing the first said pick-up units, and to the other of which the re-recorded signal train is fed after Apassing said ilnal pickup unit, and means intermittently effective for causing the re-recording units to apply the entire signal train to said medium in a manner such that predetermined sections thereof are arranged in transposed order.

l@ nais in transposed order, and means to feed the signals so transposed continuously to said final pick-up unit.

8. The method of rendering a radio communication secret which comprises recording signals on a constantly moving linear medium, collecting said signals intermittently and simultaneously at a plurality of uniformly spaced points past 4which said medium travels, causing the signals collected at each of said points to be re-recorded in transposed order at other uniformly spaced points past which said medium travels, erasing the original record of said signals at a point of travel of said medium between the leading point of collection and the trailing point of re-recording, collecting the re-recorded signals, and transmitting the same to a remote station.

9. The method according to claim V8 and; including the steps of receiving said signals at said remote station, performing the stated steps of recording, collection, erasure, re-recording in transposed order, and nal collection oil said signals at said remote station the same as at the transmitting station, and producing a response f to said signals as finally collected, said response being a restoration of the original signals by reverse transposition.

10. The method according to claim 8 in which 'the steps of recording, collection, erasure and rein the re-recording of distinct sectional portions arranged at uniformly spaced distances along said tape, a similar group of magnetizing units serially arranged at the same uniformly spaced distances along a further portion of said tape, two wipe-out magnets positioned at points along said tape, one of said points being between the group of pick-up units and the group of magnetizing units, and the other of said points being positioned ahead of said single magnetizing unit. a final pick-up unit positioned to respond to signais stored on said tape by said group of magnetizing units, means for causing said group of magnetizing units to operate simultaneously and intermittently, each under control of a respective one of said group of pick-up units, thereby to record sections oi' the originally recorded sigof said signals.

12. The method of reducing a cryptographically transmitted train of signals to intelligible form which comprises recording said signals on a constantly moving linear medium, collecting said signals intermittently and simultaneously at a plurality of uniformly spaced points past which said medium travels, causing the-signals collected at each of said points to be re-recorded in transposed order at other uniformly spaced points past which said medium travels. erasing the original record of said signals at a point of travel of said medium between the leading point of collection and the trailing point of re-recording, collecting the re-recorded signals and producing a response to said signals such that their intelligible purport is restored as before reduction to a cryptographic form.

JAlidliliiv ERNEST SMITH. 

